1. Field of the Invention
The present invention relates to a vibration motor for generating vibration by rotating an eccentric weight, and more particularly, to a bar-type vibration motor capable of miniaturizing itself by improving a support structure of a rotary shaft, a coupling structure of a stationary member and the rotary shaft, and a contacting structure of a commutator and a brush.
2. Description of the Related Art
As portable communication instruments generally used at present, mobile phones have various signal-generators to transmit various signals to users.
In other words, when messages or calls are received, the signal-generators generate sound, light or vibration so that users can know incoming of messages or calls.
The signal-generators are generally adopted as sound generators for generating bell sound or melody, illumination devices using lamps and vibrators generating vibration.
Among the signal-generators, the vibrators have various vibration motors as vibration sources, in which the vibration motors are usually classified into flat type vibration motors and bar-type vibration motors according to their configurations.
A flat type vibration motor is also called a coin-type vibration motor because it is shaped as a thin coin, and a bar-type vibration motor is also called a cylinder type vibration motor because it has a cylindrical configuration.
Both the flat type vibration motor and the bar-type vibration motor are operated based on electromagnetic induction regardless of their configurations.
The electromagnetic induction is a phenomenon in which electromagnetic force is generated across a magnetic field, when current is flown through a conductor placed perpendicular to the magnetic field.
The vibration motor converts electric energy into mechanical energy on the basis of the electromagnetic induction and generates vibration from the mechanical energy.
FIG. 1 illustrates a conventional bar-type vibration motor that will be described hereinafter.
As shown in FIG. 1, the bar-type vibration motor 100 is comprised of a stator unit 110, a rotor unit 130 and a power supply unit 150. The stator unit 110 includes a body 112 and a magnet 114.
As shown in FIG. 1 and FIG. 2, the body 112 has a support tube 112a formed integrally therein thereby forming a double-pipe structure.
In other words, the body 112 is connected at one end with the support tube 112a, and opened at the other end.
On the other hand, a bearing insert groove 112b is formed on a front end of the support tube 112a that is connected with the one end of the body 112, and the magnet 114 is attached on the outer surface of the support tube 112a so that the magnet 114 is placed in the body 112.
Next, the rotor unit 130 will be explained.
The rotor unit 130 includes an eccentric weight 131, a rotary shaft 132, a commutator 134 and an armature 136.
The eccentric weight 131 is fixed at one end of the rotary shaft 132, and a stationary member 138 is fixed at the other end of the rotary shaft 132.
The commutator 134 having several separate segments is attached on the side of the stationary member 138.
The stationary member 138 has a cylindrical projection 138a extruded from the side of the stationary member 138.
Therefore, the commutator 134 of the separate segments is attached on the side of the stationary member 138 to surround the periphery of the projection 138a. 
As above mentioned, the stationary member 138 is attached on the other end of rotary shaft 132, and the armature 136 is attached on the periphery of the stationary member 136. The armature 136 is electrically connected with the commutator 134.
The armature 136 may be of a structure which is coiled by a wire or includes a coil (not shown).
On the other hand, the rotary shaft 132 is inserted into the support tube 112a, and both ends thereof are rotatably supported by a first bearing 102 inserted in the bearing insert groove 112b and a second bearing 104 inserted into a rear end of the support tube 112a, respectively.
Next, the power supply unit 150 will be explained.
The power supply unit 150 includes a fixing cap 152 and a pair of brushes 154 installed in the fixing cap 152.
As the fixing cap 152 is coupled with the other end of the body 112, the brushes 154 touch the commutator 134 surrounding the periphery of the projection 138a. 
Power is applied from a power source to the brushes 154 through lead wires connected with the brushes 154.
If power is applied to the brushes 154 as above, voltage is supplied to the armature 136 through the commutator 134 touched by the brushes 154.
Therefore, the electromagnetic induction between magnet 114 fixed on outer surface of the support tube 112a and the armature 136 applies torque to the armature 136.
As the rotary shaft 132 is rotated by the torque generated as above mentioned, vibration is generated by the rotation of the eccentric weight 131 fixed at one end of the rotary shaft 132.
However, the conventional bar-type vibration motor 100 has following problems.
When impact is applied to a mobile phone equipped with the conventional bar-type vibration motor 100 (for example when the mobile phone is dropped), the impact is transferred to first and second bearings 102 and 104 through the rotary shaft 132.
Then, in the first and second bearing 102 and 104 supporting the rotary shaft, larger impact is applied to the first bearing 102, disposed more adjacent to the eccentric weight 131, thereby causing a problem of frequently changing the inside diameter of the first bearing 102.
When vibration is also normally generated by the rotation of the rotary shaft 132 on the basis of the electromagnetic induction, unbalanced abrasion occurs also so that the first bearing 102 is worn away more rapidly than the second bearing 104.
This happens because the impulsive load by the eccentric weight 131 is more applied to the first bearing 102, and the unbalanced abrasion shortens the life span of the bar-type vibration motor by the deformation of the bearing and generates unnecessary noises in rotating the rotary shaft 132,
As a solution to the above problem, there was proposed an approach for increasing the length of a bearing (first bearing), on which bigger load is exerted.
That is, this approach increases the depth of the bearing insert groove 112b formed in one end of the body 12 and inserts a longer bearing or several bearings into the bearing insert groove 112b, in order to reduce deformation or unbalanced abrasion of the bearings brought by impact.
But, if the depth of the bearing insert groove 112b is increased to increase the length of the bearing inserted into the bearing insert groove 112b as above, the length of the magnet 114 is to be reduced in aproportion to the reduction of a space in the body 112. This brings a problem of degrading the performance of the vibration motor by the reduction of an area for forming a magnetic field.
Therefore, because the vibration motor is to be sized up in order not to reduce the magnetic field formation area, this approach is rarely applied to the miniaturization of the current vibrator.
Also, because the structure of the body 112 as above fail to have an interspace between the eccentric weight 131 and the body 112, additional fixing elements are needed to attach the conventional bar-type vibration motor 100 on the mobile phone etc.
Also, as shown in FIG. 1 and FIG. 2, because the rotary shaft 132 is fixedly inserted into the stationary member 138, the thickness of the stationary member 138 should be increased to improve axial coupling force between the rotary shaft 132 and the stationary member 138.
Further, as shown in FIG. 2, it is difficult to miniaturize the vibration motor, because a projection 138a is formed on the side of the stationary member 138 to contact the brushes 154 with the commutator 134.
Also, because the commutator 134 is divided into several segments, sparks are generated between the commutator 134 and the brushes 154, when the brushes 154 touch the segments from one to other.
Unfortunately, the sparks occuring as above damage the commutator 134 or the brushes 154.